Recently, a reinforcing fiber material is composited with various matrix resins, and fiber-reinforced plastics obtained therefrom are widely used for various fields and applications. In particular, in an aerospace field or a general industry field in which high mechanical characteristics, heat resistance, or the like are required, a thermosetting resin like an unsaturated polyester resin, an epoxy resin, and a polyimide resin has been conventionally used as a matrix resin. However, particularly in an aerospace field, such a matrix resin has a problem of poor impact resistance due to brittleness, and thus an improvement is needed therefor. Further, in the case of a thermosetting resin, when a prepreg is prepared by using it, there are problems relating to storage and management due to short resin lifetime, and also insufficient follow-up capability to a product shape and low productivity caused by long molding time.
On the other hand, when a prepreg is prepared by using a thermoplastic resin, the impact resistance is excellent if it is provided as a composite material and storage and management of a prepreg is easy and cost for molding may be saved due to short molding time.
For improving the mechanical characteristics of a molded body in which a thermoplastic resin prepreg is used, there is a method of increasing interface contact between reinforcing fibers and a thermoplastic resin during impregnating a thermoplastic resin in continuous reinforcing fibers in the process of producing a molding material, and as a result, dynamic properties of a molded body are eventually improved. For example, there is a method of providing a reactive functional group by performing a surface oxidation treatment of reinforcing fibers, thereby improving adhesiveness between the reinforcing fibers and a thermoplastic resin. However, in such a case, when a surface treatment amount of reinforcing fibers is high, the strength of reinforcing fibers itself is impaired, causing a problem that dynamic properties of a molded body are affected. Accordingly, a method of adding a component for enhancing adhesiveness other than reinforcing fibers and a thermoplastic resin is tried. Regarding a component for enhancing adhesiveness, a method has been suggested in which an epoxy compound having high affinity for reinforcing fibers and reactivity with a thermoplastic resin is used (see, Patent Document 1). There has been also an attempt to improve adhesiveness to a polyamide resin by using a reaction product between an epoxy compound and a hydroxy group-containing amine compound as a sizing agent (see, Patent Document 2). However, in such a case, as the sizing agent has good affinity for reinforcing fibers but also has reactivity with a thermoplastic resin, the impregnation property of a thermoplastic resin into continuous fibers is not sufficient in accordance with an increase in viscosity of a matrix resin. For such reasons, there is a high possibility that tiny voids remain during manufacture of a prepreg. Furthermore, a molded body using a prepreg is generally molded by press molding or the like, but there is a limit of pressure capable of suppressing resin flow or fiber meandering. For such reasons, there is a possibility that tiny voids derived from a prepreg remain in a molded body which has been molded at such pressure, and thus a problem arises that a molded body having excellent dynamic properties cannot be obtained. Furthermore, although there is an example that a sizing agent with excellent impregnation property is added, it is an example only relating to a thermoplastic resin with low viscosity and no determination is made regarding a thermoplastic resin with high viscosity (see, Patent Document 3). Taken together, in a current state, a sizing agent having affinity for both of reinforcing fibers and thermoplastic resin and also satisfying the impregnation property of a thermoplastic resin into reinforcing fibers have not been known.